The invention generally relates to a radiation imaging apparatus and more particularly to a scintillation camera which has improved sensitivity and performance.
The scintillation camera is an instrument which gives an image of radionuclide distribution in a particular human organ or an local area thereof. The radionuclide is given to the human body usually by intravenous injection and taken up by a particular area of the human body. The camera includes a scintillator, which is actuated by the gamma rays emitted by the radionuclide to produce scintillations. A plurality of photoelectric elements arranged in close optical relation to the scintillator produce an electrical output in response to each of the scintillations.
A suitable electronic device translates the outputs of the photoelectric elements caused by each scintillation into electrical signals (to be referred to as the position signals or the signals X and Y) corresponding to the coordinate (x,y) positions where the scintillations have occurred in the scintillator and an electrical signal (to be referred to as the energy signal or the signal Z) whose pulse height corresponds to the energy level of each scintillation. If the energy signals produced for a certain period of time are plotted on a graph with the counted number taken on the ordinate and the energy level on the abscissa, an energy histogram or spectrum will be obtained, which has a single or a plurality of peaks depending upon the kind of radionuclide. The peak is commonly referred to as the photoelectric peak or photopeak.
The energy signal is applied to a pulse height analyzer, which checks if the level of the signal falls within a predetermined range defined by a high and a low signal level. The range will be referred to as the energy window. If the energy signal has a pulse height within the window, an image point is depicted on a CRT display in accordance with the calculated coordinate position of the scintillation in the scintillator. The image point is brighter at a position or picture element where the counted number of energy signals is higher and darker at a position or picture element where the counted number is lower.
Theoretically, the energy signals should have the same pulse height regardless of the position at which the radiations causing the signals strike the scintillator provided that the radiations have the same energy level. In other words, the energy histogram should have the same photopeak at all positions in the scintillator. Practically, however, this is not true, but it is inevitable that the photopeak of the energy histogram varies with the position where the scintillation occurs in the scintillator. The phenomenon may be referred to as the dependency of the energy signal on the position of the scintillation that has caused the energy signal, and more briefly, the position-dependency of the energy signal.
If one and the same window is applied to all energy signals caused by the scintillations that have occurred at different positions of the scintillator, the sensitivity of the camera is not uniform but differs at different parts of the scintillator.
Correction of the nonuniformity of sensitivity can be effected by changing the energy window for each of the coordinate positions in the scintillator in accordance with the photopeak shift of the energy histogram in each of the coordinate positions.